A magnetic recording medium comprising a non-magnetic support having thereon a magnetic layer containing acicular crystalline ferromagnetic particles such as .alpha.-Fe.sub.2 O.sub.3 or CrO.sub.2 in a binder is conventionally used. Recently, there has been a demand for a magnetic recording medium with minimal size but with increased capacity for recording information. In this field, therefore, improvements with respect to higher density recording of a magnetic recording medium have become an important subject. In this connection, the maximum dimension of acicular particles must be far smaller than the recording wavelength or recording bit length in order to obtain a magnetic recording medium which uses conventional acicular ferromagnetic particles but which is also suitable for higher density recording. Currently, improvements have produced acicular ferromagnetic particles having a particle diameter of about 0.3 .mu.m, and the shortest recording wavelength achieved has been about 1 .mu.m. Yet, the dimension of acicular ferromagnetic particle should be even smaller, if a magnetic recording medium suitable for much higher density recording is needed. However, in such acicular magnetic particles having such a small size, the width thereof is so narrow, that is, 100 .ANG. or less, and the volume thereof is so small, that is, 1.times.10.sup.-17 cm.sup.3 or lower, that there is also a problem that, as a result of heat disturbance and effect of surface (i.e., the effect in that the magnetic characteristics in surface portions of a magnetic medium are reduced because the spin of atoms which is present at vicinity of surface of the magnetic medium becomes unstable), magnetic characteristics decrease and magnetic orientation can not be successfully carried out.
To solve the above problem, a magnetic recording medium using ferromagnetic particles of tabular hexagonal crystal ferrite having an axis of easy magnetization perpendicular to the plate has recently been developed, as disclosed in JP-A-58-6525 and JP-A-58-6526. (The term "JP-A-" as used herein means an "unexamined published Japanese patent application"). Those ferromagnetic particles have made it possible to achieve an average particle size of 0.05 .mu.m or less and thus high density recording has become possible using those ferromagnetic particles.
Thus, ferromagnetic particles can be granulated more finely, and the packing density thereof can be increased by the use of such hexagonal crystal ferrite ferromagnetic particles, whereby output and C/N (carrier/noise ratio) of the magnetic recording medium can be improved.
However, deposite this achievement, it has become extremely difficult to maintain the durability of a magnetic layer, as the granulation of particle size of ferromagnetic particles proceeds. There have been proposed many approaches to avoid the above problem. For instance, a magnetic layer is made tough and noise is decreased by using isophorone type hardening agents (as described in JP-A-60-55516), S/N is improved by determining the active hydrogen equivalent in a binder (as described in JP-A-60-70518), the dispersibility of each component in a magnetic layer is improved by mixing CBA (cellulose butylacetate), polyurethane resins and NBR (nitrile-butadiene rubber) in combination (as described in JP-A-60-193127), abrasive agents are added under predetermined conditions (as described in JP-A-61-94222), reproduced output is improved by adding graphited carbon (as described in JP-A-61-139926), the durability of the magnetic layer is improved by emulsifying a binder (as described in JP-A-61-210517), the durability of a magnetic layer is imporved by decreasing .mu. value (friction coefficient) of graphite to be added (as described in JP-A-61-214128), efficiencies of manufacturing steps are improved by using radiation sensitive binders (as described in JP-A-61-233414), particle sizes of abrasive agents are reduced (as described in JP-61-A-273735), a binder is used in an amount of from 10 to 30 wt % (as described in JP-A-61-289522), and the specific surface area of magnetic particles is defined to be from 70 to 120 m.sup.2 /g (as described in JP-A-62-38531). But the above approaches are not yet satisfactory, because good results could not be obtained in the thermocycle durability test of a floppy disk (i.e., continuous running durability test under the recycle of conditions at a high temperature of 60.degree. C. and a low temperature of 5.degree. C. each for 24 hours.).